Method and apparatus for forming solder bumps

ABSTRACT

The present invention discloses a method and apparatus for forming solder bumps by a molten solder screening technique in which a flexible die head constructed of a metal sheet is utilized for maintaining an intimate contact between the die head and a solder receiving mold surface. The flexible die head, when used in combination with a pressure means, is capable of conforming to any curved mold surface as long as the curvature is not more than 2.5 μm per inch of die length. The present invention further provides a method and apparatus for filling a multiplicity of cavities in a mold surface by providing a stream of molten solder and then intimately contacting the surface of the molten solder with a multiplicity of cavities such that the molten solder readily fills the cavities. The apparatus further provides means for removing excess molten solder from the surface of the mold without disturbing the molten solder already filled in the cavities. The present invention further discloses a flexible die for dispensing molten solder consisting of a die body that is constructed of a metal sheet capable of flexing of not less than 1.5 μm per inch of die length, a gate opening for receiving a supply of molten solder, a slot opening for dispensing the molten solder onto a solder receiving surface, and a pressure means associated with the die body for providing adequate pressure such that the die body intimately contacting the solder receiving mold surface.

FIELD OF THE INVENTION

The present invention generally relates to a method and apparatus forforming solder bumps on an electronic structure and more particularly,relates to a method and apparatus and method for forming solder bumps ona semiconductor chip by a molten solder screening technique.

BACKGROUND OF THE INVENTION

In modern semiconductor devices, the ever increasing device density anddecreasing device dimensions demand more stringent requirements in thepackaging or interconnecting techniques of such devices. Conventionally,a flip-chip attachment method has been broadly used in the packaging ofIC chips. In the flip-chip attachment method, instead of attaching an ICdie to a lead frame in a package, an array of solder balls is formed onthe surface of the die. The formation of the solder balls is normallycarried out by an evaporation method of lead and tin through a mask forproducing the desired alloy balls. More recently, the technique ofelectro-deposition has been used to produce the solder balls inflip-chip packaging.

Other solder ball formation techniques that are capable ofsolder-bumping a variety of substrates have also been proposed. Thesetechniques work fairly well in bumping semiconductor substrates thatcontain solder structures over a minimum size. One of the more popularlyused techniques is a solder paste screening technique which can be usedto cover the entire area of an 8 inch wafer. However, with the recenttrend in the miniaturization of device dimensions and the reduction inbump-to-bump spacing (or pitch), the solder paste screening techniquehas become impractical. For instance, one of the problems in applyingsolder paste screening technique to modern IC devices is the pastecomposition itself. A paste is generally composed of a flux and solderalloy particles. The consistency and uniformity of the solder pastecomposition therefore become more difficult to control with a decreasingsolder bump volume. A possible solution for this problem is theutilization of solder pastes that contain extremely small and uniformsolder particles. However, this can only be done at a high cost penalty.Another problem in using the solder paste screening technique in modernhigh density devices is the reduced pitch between bumps. Since there isa large reduction in volume from a screened paste to the resultingsolder bump, the screen holes must be significantly larger in diameterthan the final bumps. Thus stringent dimensional control of the bumpsmakes the solder paste screening technique impractical for applicationsin high density devices.

A more recently developed injection molded solder (IMS) techniqueattempted to solve these problems by dispensing molten solder instead ofsolder paste. However, problems have been observed when the technique isimplemented to wafer-sized substrates, U.S. Pat. No. 5,244,143, assignedto the common assignee of the present invention, discloses the injectionmolded solder technique and is hereby incorporated by reference in itsentirety. One of the advantages of the injection molded solder techniqueis that there is very little volume change between the molten solder andthe resulting solder bump. The IMS technique teaches the use of a twoinch wide head that fills boro-silicate glass molds that are wide enoughto cover most single chip modules. A narrow wiper provided behind thesolder slot passes the filled holes once to remove excess solder.However, when a two inch wide head is used to fill molds for largewafers, i.e., such as an 8 inch or 12 inch wafer, the fill requires atleast four or six successive scans by the head. During such successivescans, the overlapped areas between scans inevitably have degraded fillcharacteristics such as solder streaks between holes and non-uniformfills.

Another disadvantage of the IMS technique is the mold flatness and thehead flatness. The boro-silicate glass molds used are typically thinenough to allow some flexibility over a length of 8 or 10 inches. At atypical thickness of 1/16", the large scale flexibility of the moldcauses the mold to conform to the contour of the support-plate thatholds the mold. During a relatively fast heating and cooling of thesupport-plate in the solder-fill process, the support-plate deforms overa large wafer-sized area. Under the IMS head pressure, the mold conformsto the support-plate contour and therefore becomes curved over itsentire width. When compressed by a rigid IMS head, there is a highlikelihood that a gap will be formed between the head and the mold. Thegap causes poor wiping of excess solder from the mold surface resultingstreaking and poor filling problems. Furthermore, the IMS techniquerequires vacuum to induce a solder flow by generating a negativepressure at the leading edge of the solder slot. The molten solder willleak into the vacuum slot when the gap between the vacuum and the solderslot is larger than a maximum allowable value, typically 5 μm. Moreover,when both the mold and the head are made of glass material, the frictiongenerated by glass sliding on glass causes a significant drag on thescanning head. Any hard debris on the mold surface may also causesignificant damage to the mold.

It is therefore an object of the present invention to provide a methodfor forming solder bumps by a molten solder screening technique thatdoes not have the drawbacks and shortcomings of the conventional solderbumping techniques.

It is another object of the present invention to provide a method forforming solder bumps by a molten solder screening technique that doesnot require the use of a vacuum source and a vacuum slot in the moldhead.

It is a further object of the present invention to provide a method forforming solder bumps by a molten solder screening technique wherein amolten solder die of sufficient length to cover the entire area of alarge wafer is used.

It is another further object of the present invention to provide amethod for forming solder bumps by a molten solder screening techniquein which a flexible die head capable of conforming to an uneven moldsurface is used.

It is still another object of the present invention to provide a methodfor forming solder bumps by a molten solder screening technique in whichfresh, un-oxidized molten solder is used for each mold fill.

It is yet another object of the present invention to provide a methodfor forming solder bumps by a molten solder screening technique in whicha pressure means is used in combination with a flexible die toaccommodate glass molds with large curvatures.

It is still another further object of the present invention to providean apparatus for forming solder bumps by a molten solder screeningtechnique in which a mechanical support means is used for engaging amold with a flexible die such that a predetermined pressure ismaintained between the mold cavities and the surface of a molten solderstream.

It is yet another further object of the present invention to provide anapparatus for forming solder bumps by a molten solder screeningtechnique wherein an excess solder removal means is used to removeexcess molten solder from the surface of the mold.

It is still another further object of the present invention to providean apparatus for forming solder bumps by a molten solder screeningtechnique in which a mold constructed of a material that has acoefficient of thermal expansion substantially similar to that ofsilicon or the final solder receiving material is used.

SUMMARY OF THE INVENTION

The present invention discloses a method and apparatus for formingsolder bumps by a molten solder screening technique in which a flexibledie member is used in combination with a pressure means to enable thedie member to intimately engage a mold surface and thus filling the moldcavities for forming the solder bumps.

In a preferred embodiment, a method for filling a multiplicity ofcavities positioned in the surface of a substrate with molten solder isprovided which can be carried out by the steps of first providing astream of molten solder, then passing the multiplicity of cavities inthe surface of the substrate in intimate contact with the surface of themolten solder stream, the contact is adjusted such that molten solder inthe stream exerts a pressure against the surface of the substrate so asto fill the multiplicity of cavities with the molten solder, and thenremoving the excess molten solder from the surface of the substrate. Oneof such suitable substrates is a mold.

The present invention is also directed to an apparatus for filling amultiplicity of cavities positioned in the surface of a substrate with amolten solder which includes a stream of molten solder, a mechanicalsupport means for engaging the multiplicity of cavities in the surfaceof the substrate with a surface of the stream such that a predeterminedpressure is maintained between the multiplicity of cavities and thestream surface, and a solder removal means for removing excess moltensolder from the surface of the substrate.

In another preferred embodiment, a method for forming a multiplicity ofsolder bumps on the surface of an electronic device is provided whichcan be carried out by the operating steps of first providing a substratethat has a multiplicity of cavities in a top surface, then providing astream of molten solder, then passing the multiplicity of cavities inthe surface of the substrate over and intimate contacting the surface ofthe stream of molten solder, the contact can be adjusted such thatmolten solder in the stream exerts a pressure against the surface of thesubstrate so as to fill the multiplicity of cavities with the moltensolder, then removing the excess molten solder from the surface of thesubstrate and allowing the molten solder in the multiplicity of cavitiesto solidify, then contacting and transferring the solder bumps in themultiplicity of cavities with and to a solder receiving surface of anelectronic device.

The present invention is further directed to a die for dispensing moltensolder that includes a die body of elongated shape formed of a metalsheet non-wetting to solder capable of flexing of no less than 1.5 μmper inch of die length, the die body has a front side and a back side, agate opening through the front and back sides of the die body forreceiving molten solder from a reservoir, an elongated slot opening inthe front side of the die body in fluid communication with the gateopening for distributing molten solder to a solder receiving surface,and a pressure means mounted on the back side of the die body forproviding pressure to the die body such that the die body intimatelycontacts the solder receiving surface.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages of the presentinvention will become apparent from the following detailed descriptionand the appended drawings in which:

FIG. 1A is a schematic illustrating the present invention flexible diehead in a side view.

FIG. 1B is a schematic illustrating the present invention die head in abottom view.

FIG. 2 is a schematic illustrating the present invention die headpositioned over a wafer-sized mold.

FIG. 3A is a schematic showing the present invention flexible die headengaging a convex mold surface.

FIG. 3B is a schematic illustrating the present invention flexible diehead engaging a concave mold surface.

FIG. 3C is a schematic illustrating the present invention flexible diehead engaging a complex-shaped mold surface.

FIG. 4A is a top view of the present invention flexible die head havinga pressure means of springs mounted thereto.

FIG. 4B is a side view of the present invention flexible die head havinga pressure means of springs mounted thereto.

FIG. 5A is a side view of the present invention flexible die head havinga pressure means of bellows mounted thereto.

FIG. 5B is a bottom view of the present invention flexible die headhaving a pressure means of bellows mounted thereto.

FIG. 6 is a schematic illustrating a side view of the present inventionflexible die head that is globally flexible.

FIG. 6A is a partially enlarged, cross-sectional view of FIG. 6illustrating the engagement between the flexible die head and the mold.

FIG. 7A is a cross-sectional view of the present invention soldertransfer apparatus.

FIG. 7B is a cross-sectional view of the present invention mold havingsolder bumps filled therein intimately engaging a solder receivingsurface on an electronic device.

FIG. 7C is a cross-sectional view of the solder receiving surface on theelectronic device after the solder bumps are transferred thereto.

FIG. 7D is a cross-sectional view of the solder receiving surface on theelectronic device and the solder bumps after a reflow process.

FIG. 8 is an illustration of the present invention solder bump moldingprocess utilizing a flexible die head and a moving face-up mold.

FIG. 9A is an illustration of the present invention flexible die headhaving a pressure means of bellows in an uncompressed state.

FIG. 9B is an illustration of the present invention flexible die headbeing compressed by a pressure means of bellows onto a face-down mold.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention discloses a method and apparatus for formingsolder bumps by a molten solder screening technique in which a flexibledie head is pressed against a mold surface by a pressure means forachieving an intimate contact between a die opening and a multiplicityof cavities in the surface of the mold. The flexible die head alsoserves a desirable function of a wiper by utilizing its trailing edge asa removal means for removing excess molten solder from the surface ofthe mold.

The present invention also provides a method for filling a multiplicityof cavities in the surface of a mold with molten solder by providing astream of molten solder and then passing a multiplicity of cavities inthe mold surface in contact with the surface of the stream by adjustinga contact force such that the molten solder in the stream exerts apressure against the surface of the mold to fill the cavities withsolder and then removing excess solder from the surface of the mold. Thestream of molten solder is supplied through a die head constructed of aflexible metal sheet that is capable of flexing at least 1.5 μm per inchof die length. The stream of molten solder has a composition betweenabout 58% tin/42% lead and about 68% tin/32% lead. The multiplicity ofcavities each has a depth-to-width aspect ratio of between about 1:1 andabout 1:10. The mold body is made of a material that has a coefficientof thermal expansion substantially similar to that of silicon or thefinal solder receiving material. The contact between the multiplicity ofcavities and the surface of the molten solder stream can be adjusted bya pressure means exerted on the flexible die.

The present invention also discloses an apparatus which can be used tofill a multiplicity of cavities in a mold surface by a molten solderwhich includes a stream of molten solder, a pressure means (or amechanical support device) for engaging the multiplicity of cavities inthe surface of the mold with a surface of the stream such that apredetermined pressure is maintained between the multiplicity ofcavities and the surface of the stream, and a solder removal means forremoving excess molten solder from the surface of the mold. Theapparatus may further include a flexible die member for flowing thestream of molten solder therethrough. The solder removal means forremoving excess solder from the surface of the mold is a wiper formed bythe trailing edge of the flexible die.

The present invention further discloses a method for forming amultiplicity of solder bumps on the surface of an electronic device byfirst providing a mold that has a multiplicity of cavities formed in atop surface and contacting the cavities with a flexible die head throughwhich a stream of molten solder flows, and then filling the cavitieswith the molten solder and transferring the solder bumps to a solderreceiving surface on an electronic device by intimately contacting themultiplicity of cavities with the solder receiving surface and heatingto a reflow temperature of the solder. The stream of molten solder isprovided at a pressure of not less than 2 psi. The solder receivingsurface of the electronic device may be a surface of a semiconductorchip having bond pads formed thereon.

The present invention further provides a die for dispensing moltensolder that is constructed of a die body of elongated shape formed of ametal sheet capable of flexing of not less than 1.5 μm per inch of dielength, the die body has a gate opening and an elongated slot opening toallow a molten solder to flow therethrough, and a pressure means mountedon the back side of the die body for providing pressure to the die bodyand forming an intimate contact with a solder mold. The pressure meansmay be a plurality of springs that are mounted in a perpendiculardirection to the elongated die body, or a plurality of bellows that aremounted in a perpendicular direction to the elongated die body. The diebody further includes a trailing edge acting as a wiper for removingexcess molten solder from the solder mold.

Referring now to FIG. 1A, wherein a present invention flexible die head10 for forming solder bumps by a molten solder screening process isshown. The die head 10 has a die body 12 which is made of a thin,flexible metal sheet such as stainless steel or any other suitablematerial non-wetting to solder. The die body 12 has a gate opening 14and a slot opening 16. The gate opening 14 has a narrow width and isgenerally positioned at the center of the die body 12. The gate opening14 provides fluid communication between a front side 18 of the die body12 and a back side 22 of the die body 12. The gate opening 14 furtherprovides a passageway for receiving a molten solder 24 stored in asolder reservoir 26. The molten solder 24 is kept under an inert gasatmosphere at a pressure of approximately 5 psi. A suitable inert gasused is nitrogen, i.e., as shown in FIG. 1A, supplied by a nitrogensource 28. The functions of the inert gas in the solder reservoir aretwo fold. First, it provides an inert gas blanket over the solder 24such that any oxidation of the molten solder 24 can be avoided.Secondly, the nitrogen blanket in the reservoir 26 provides a positivepressure such that molten solder 24 flows easily through the gateopening 14 into the slot opening 16. In continuous operation, thenitrogen pressure is turned off when flexible die head 10 moves betweenmolds to prevent solder loss. The slot opening 16 is provided in thefront surface 18 of the die body 12 in a suitable depth such that moltensolder 24 can easily flow into the mold cavities 32 (shown in FIG. 2).The width of the slot opening 16 is predetermined such that it coverssubstantially all the cavities 32, 38 in the mold surface 42. Theopening 16 is also wide enough to cover the entire width of a wafersurface to be later bumped by first filling a mold surface having thesame width.

The die body 12 also functions as a high temperature squeegee whichseparates the molten solder in the slot opening 16 from the moltensolder filled in the mold cavities 32 (FIG. 2). In order to accomplishthis task, the die body 12 must be positioned closely behind a moltensolder flow front when the flow front completely covers a linear areaacross the mold surface 42. The aspect ratio (the depth-to-width ratio)of the mold cavities 32 are typically 0.5 so that solder flows easilyinto and penetrates to the bottom of the cavity. It has been noted inthe present invention novel method that, at this point, it is criticalto "cut" or "sever" the molten solder as the mold plate 34 scans overthe molten solder flow. This difficult task is accomplished by thepresent invention flexible die body 12 in a unique unexpected mannersince the trailing edge 36 of the die body 12 functions as a flexiblewiper, or squeegee, to continuously scrape the surface 42 of the moldplate 34. FIG. 1B illustrates a bottom view of the flexible die head 10shown in FIG. 1A. The trailing edge 36 of the die body 12 thereforeeffectively "cuts" the solder supply 24 from the molten solder that hasalready been deposited in the mold cavities 32. The trailing edge 36,should be sufficiently smooth to assure a uniform contact across theoptically-smooth mold surface 42. It is yet another unique unexpecteddiscovery in the present invention method that the trailing edge 36, orthe wiper, of the die body 12 is flexible only on a global scale, i.e.on a scale of the width of the mold plate 34. As a result, the trailingedge 36 does not enter into cavities 32 and damage the solder bumpsformed in the cavities. (shown in FIG. 2). The width of the slot opening16 is predetermined such that it covers substantially all the cavities32, 38 in the mold surface 42. The opening 16 is also wide enough tocover the entire width of a wafer surface to be later bumped by firstfilling a mold surface having the same width.

The die body 12 also functions as a high temperature squeegee whichseparates the molten solder in the slot opening 16 from the moltensolder filled in the mold cavities 32 (FIG. 2). In order to accomplishthis task, the die body 12 must be positioned closely behind a moltensolder flow front when the flow front completely covers a linear areaacross the mold surface 42. The aspect ratio (the depth-to-width ratio)of the mold cavities 32 are typically 0.5 so that solder flows easilyinto and penetrates to the bottom of the cavity. It has been noted inthe present invention novel method that, at this point, it is criticalto "cut" or "sever" the molten solder as the mold plate 34 scans overthe molten solder flow. This difficult task is accomplished by thepresent invention flexible die body 12 in a unique unexpected mannersince the trailing edge 36 of the die body 12 functions as a flexiblewiper, or squeegee, to continuously scrape the surface 42 of the moldplate 34. FIG. 1B illustrates a bottom view of the flexible die head 10shown in FIG. 1A. The trailing edge 36 of the die body 12 thereforeeffectively "cuts" the solder supply 24 from the molten solder that hasalready been deposited in the mold cavities 32. The trailing edge 36,should be sufficiently smooth to assure a uniform contact across theoptically-smooth mold surface 42. It is yet another unique unexpecteddiscovery in the present invention method that the trailing edge 36, orthe wiper, of the die body 12 is flexible only on a global scale, i.e.on a scale of the width of the mold plate 34. As a result, the trailingedge 36 does not enter into cavities 32 and damage the solder bumpsformed in the cavities. The word "flexibility" used in the context ofthe present invention is on the scale of inches, while the word"rigidity" used in the context of the present invention is on the scaleof thousandths of an inch or mils.

As shown in FIG. 2, the present invention flexible die body 12 scanssmoothly over the surface 42 of the mold plate 34, i.e., over the top ofall the cavities 32 allowing the solder within the cavities to staywhile removing excess solder from the surface 42. This operationcontinues as the mold plate 34 is scanned over the molten solder supply24 until all the cavities are filled. As shown in FIG. 2, the cavities38 not yet scanned over die body 12 are still empty. The presentinvention novel method only requires the die body 12 to pass over themold plate 34 once for a complete fill. The novel process thereforeeliminates solder streaking and non-uniform fill problems caused bymultiple scannings with overlapped areas encountered in conventionalmethods.

Referring now to FIGS. 3A˜3C, wherein the present invention flexible diehead 10 is shown fitted to various contoured molds 44, 46 and 48. Thedetails of the flexible die head 10, i.e., the gate opening, the slotopening and the trailing edge (or wiper) are not shown for simplicityreasons. The mold 44, 46 and 48 are also shown greatly simplified withonly the mold surfaces shown. For instance, FIG. 3A shows aconvex-shaped mold surface on mold 44, FIG. 3B shows a concave-shapedmold surface on mold 46 and FIG. 3C shows a complex-shaped mold surfacefor mold 48. FIGS. 3A, 3B and 3C therefore demonstrates the desirablecharacteristic of the present invention novel flexible die head 10 ofits capability to conform to any curved mold surface, as long as thecurvature is within an allowable maximum value. It has been found that,with the proper pressure means (not shown in FIGS. 3A˜3C), the presentinvention novel flexible die head 10 is capable of flexing to not lessthan 1.5 μm per inch of die length, and preferably to not less than 2.5μm per inch of die length. The surface 18 of the flexible die head 10should be sufficiently smooth in order to assure an uniform contactacross an optically-smooth mold surface. It should again be emphasizedthat the die head is flexible only on a global scale, as previouslydefined, over the width of the mold. It remains rigid on a local scalesuch that it does not enter into individual cavities in the surface ofthe mold. As previously defined, the word "flexibility" as used in thepresent application is on the scale of inches, while the word "rigidity"as used in the present application is on the scale of mils. The presentinvention novel flexible die head 10 is therefore capable of completelytracking and linearly matching a mold curvature (within a maximumallowable curvature) as the die head scans across the surface of a mold,regardless of the shape of the mold curvature.

FIGS. 4A, 4B, 5A and 5B illustrate the present invention novel flexibledie head in combination with a pressure means. In FIGS. 4A and 4B, a topview and a side view of the present invention flexible die head 10having a pressure means 50 of a plurality of mechanical springs 52mounted thereto are shown. The pressure means 50 is attached to the diehead 12 by mechanical means 56, i.e., such as bolts. An uniform pressingforce along a full eight inch or twelve inch wide flexible die head 12can be supplied by a linear array of a plurality of springs 52. Theconstruction of the pressing means 50 can be of any convenientmechanical method. For instance, the springs 52 may be individuallyseated in blind holes 54 that are provided in the bottom surface 60 ofthe pressing means 50. It has been observed that, depending on thecurvature of the mold surface, some springs 52 may be compressed morethan the others, similar to an application of bed springs.

FIGS. 5A and 5B show a second embodiment of the pressing means 50 of aplurality of stainless steel bellows 64. The bellows 64 function both asmechanical springs and solder supply tubes wherein solder supply 66enters the bellow 64 and then exits through a plurality of gate openings14. This configuration is especially suitable when a modified wavesoldering tool is utilized to carry out the present invention method.

It has been found that molds used for a flip-chip solder bumping processusually have aspect ratios of 0.5, i.e., the depth is half the width. Atsuch shallow aspect ratios, the leading edge of incoming molten solderpushes the air out of the cavity ahead of the solder flow front. Apliable Teflon seal 70 (shown in FIG. 6A) allows the air to escape, butnot the solder. The present invention novel method therefore does notrequire the evacuation of cavities having large aspect ratios, i.e.,larger than one, as that normally required in the conventional injectionmolded solder molds, for instance, for molding solder columns in columngrid arrays (CGA). The present invention molten solder screeningtechnique is therefore suitable for filling cavities that have aspectratios smaller than one, or for filling cavities that have aspect ratiosbetween about 1:1 and about 1:10.

The present invention molten solder screening technique does not requirethe use of vacuum to evacuate cavities and to initiate a solder feed. Asa consequence, there is no need for a vacuum slot or a vacuum link. Thepresent invention novel method therefore eliminates the problem inconventional methods of a cross-leak of molten solder into a vacuumpassage. The present invention technique feeds a molten solder to a slotopening through a gate opening while the molten solder is under apressure of between about 2 psi and about 5 psi. The positive pressurecan be provided by an inert gas such as nitrogen, which is connected toa solder reservoir (as shown in FIG. 1A). Alternatively, the positivepressure can be supplied by a variable speed solder pump (not shown) asthat typically used in a wave soldering tool. To develop the positivepressure, there should be no solder bypass between the solder pump andthe flexible die head unless the desired pressure has been reached.Alternatively, a pressure safety valve may be included which opens atpressures exceeding 5 psi. The fluid communication between the solderpump (not shown) and the present invention flexible die head is achievedby supplying molten solder from a die head which is positioned under amold, as shown in FIG. 5A.

The present invention novel flexible die head which is globally flexibleand locally rigid is further illustrated in FIGS. 6 and 6A. FIG. 6A isan enlarged, cross-sectional view of a section of FIG. 6. The presentinvention novel flexible die head utilizes stainless steel as anon-wetting head material. The thickness of the die body 12 can besuitably controlled within a range between about 0.025" and about 0.045"and thus providing sufficient flexibility on a global scale of the wholewidth of a wafer. On the bottom surface 18 of the die body 12, a slotopening 16 (shown in FIG. 1A) is cut to a width of between about 1/32"and about 1/8", and preferably to a width of about 1/16". The depth ofthe slot opening 16 is cut into the die body 12 to between about 0.010"and about 0.020", and preferably to about 0.015". The length of the slotopening 16 is slightly larger than the widest part of an etched area ofa glass mold plate. The length therefore can be either about 8" or 12"depending on the size of the wafer to be solder bumped.

In an alternate embodiment of the present invention, to further assistthe sliding motion of the flexible die head on the glass mold plate andto avoid damages to the mold surface a Teflon® coating is added to thebottom surface 18 of the die body 12. A suitable thickness of theTeflon® coating is between about 0.005" and about 0.010" which includesthe thickness of an adhesive backing. After a Teflon® tape 70 is appliedto the bottom surface 18 of the die head 12, the slot opening 16 in thedie body 12 is cut into the Teflon® tape 70 such that solder may flowfreely along the entire length of the opening 16. When a Teflon® tape of0.010" thickness is used, the depth of the slot opening 16 is increasedto a total depth of approximately 0.025". It has been found that at sucha depth, molten solder flows readily along the entire length of the slotopening 16 under a positive pressure of about 3 psi-5 psi. The Teflon®tape 70 provides improved lubricity property to the surface of the diebody 12 and thus eliminating possible scratching or other damages to thesurface of the glass mold 48. The soft Teflon® tape 70 further providesthe benefit that it embeds any hard particles on the surface of theglass mold 48, and thus preventing damages to the mold surface. TheTeflon® tape 70 further provides the benefit that it reduces drag on themold surface to ensure a smooth scanning of the flexible die head.

During an excess solder removal process from the surface of the glassmold, the Teflon® tape 70 on the die body 12 does not affect the moltensolder deposited in the cavities since molten solder does not wetTeflon®. The pliable Teflon® tape 70 on the die body 12 further assiststhe removal, or the wiping, process of excess solder from the surface ofthe mold plate. A typical operating head joining pressure, i.e., thepressure between the Teflon® tape and the glass mold surface, is betweenabout 15 psi and about 25 psi which is supplied by the plurality ofsprings or bellows, as shown in FIGS. 4A˜5B. The Teflon® tape 70, eventhough pliable, is still hard enough as not to disturb, or to scoop outmolten solder from the cavities that were previously filled.

After the cavities 32, as shown in FIGS. 2 and 6A, are completelyfilled, the mold plate 48 is cooled to allow the molten solder in thecavities 32 to solidify. The mold plate 48, complete with filledcavities is then ready for the next processing step for transferringsolder bumps onto a solder receiving surface on an electronic device.This is shown in FIGS. 7A˜7D. A transfer of solder bumps to anelectronic device can be accomplished by a fixture lid 72 and a flatbase member 74. Onto the flat base member 74, it is desirable that acompressible material layer 76 such as a polymeric based foam layer isplaced to ensure an improved mating between the mold surface and thesolder receiving surface on the electronic device. The compressiblematerial layer 76 should be manufactured of a high temperature endurantmaterial, i.e., up to the reflow temperature of the solder bumps atabout 220° C. On top of the compressible material layer 76, theelectronic structure 78 is positioned with a plurality of bond pads 82in a faced-up position. Alternatively, a thin layer of a flux material(not shown) can be dispersed over the bond-pad side of the electronicstructure 78. It is possible to transfer the solder bumps 80 withoutusing a flux coating by subjecting the bumps 80 to a proper reducingatmosphere in the solder transfer furnace.

During the solder transfer process, as shown in FIG. 7A, thesolder-filled mold plate 48 is aligned to and positioned over theelectronic structure 78 with the solder bumps 80 facing thecorresponding bond pads 82. To ensure a successful operation of thepresent invention novel method, the alignment process for the solderbumps 80 to the bond pads 82 is important. The fixture lid 72 is placedon the back side of the mold plate 48 and a small compression force isapplied between the fixture lid 72 and the base member 74 such that thecompressible material layer 76 is slightly compressed. The fact that theelectronic structure 78, i.e., may be a silicon wafer, and the moldplate 48 are slightly flexible further ensures that an intimate contactbetween the two members over the entire surface area is achieved. Thefixture assembly 86 is then positioned into a reflow furnace (not shown)which is typically a belt furnace. The temperature of the fixtureassembly 86 is then increased to a temperature that is not less than thereflow temperature of the solder material, i.e., about 220° C., and thencooled down to the room temperature upon exiting from the furnace.

As shown in FIG. 7B, after the solder bump transfer process is completedin the furnace, the electronic structure 78 and the mold plate 48 areremoved from the fixture assembly 86. The mold plate 48, after cooled toa temperature below the melting point of the solder material, i.e.,about 183° C., can be removed from the surface of the electronicstructure 78. This is shown in FIG. 7C. The solder bumps 80 are nowmetallurgically bonded to the bond pads 82 on the surface of theelectronic structure 78. Alternatively, as shown in FIG. 7D, a finalreflow process of the solder bumps 80 can be conducted such that fullsolder spheres 88 are formed.

The solder material used in forming the solder bumps may be suitablyselected from a composition range between about 58% tin/42% lead andabout 68% tin/32% lead, a typical example is a eutectic 63% tin/37% leadsolder composition having a melting point of about 183° C. for use inthe present invention novel method. The present invention molten solderscreening technique may also utilize any other solder compositions,including those lead-free alloys which have become increasingly popularin various semiconductor applications.

Industrial Applicability

The present invention novel method and apparatus can be easily adaptedin various industrial fabrication methods for electronic devices. Forinstance, the method can be practiced in a stationary face-up moldmethod, in a moving face-up mold method, in a moving face-down moldmethod or in any other suitable methods.

In a stationary face-up mold method, the molds do not move and arepositioned with a surface that has etched cavities facing up. A presentinvention novel flexible die head can be used to scan across the topsurface of the mold plate. This mode of operation is relatively easy toimplement since the molds can be simple placed on a support plate thatis either heated or cooled, and then the flexible die head is used toscan across a heated mold plate. The novel flexible die head of thepresent invention consists of a central solder reservoir that is under anitrogen pressure to supply molten solder to a slot opening through agate opening under the nitrogen pressure. This has been shown in FIGS.1A˜2. The drawback of this mode of operation is that it requires thehandling of the molds before and after each solder fill by placing themonto or removing them from a heating/cooling plate, it may be laborintensive and thus costly for a manufacturing process.

In a moving face-up mold operation, mold plates move with their etchedcavity side facing up. A present invention flexible die head is mountedin a stationary position to scan over the top of the mold transported ona moving belt. This mode of operation is more compatible with amanufacturing process. Since the mold plates move relative to astationary flexible die head, the mold plates may be pre-heated as theymove closer to the solder fill location and then, cooled after thesolder fill. A schematic illustrating this mode of operation is shown inFIG. 8. Unfilled mold plates 92 are moved on a moving belt 94 through apre-heat station 96, a heating station 98 under a flexible die head 100,and then to a cooling station 102 before it is removed from the belt 94as filled molds 104. As shown in FIG. 8, this mode of operation allowsmolds to be filled sequentially as is frequently seen in manufacturingprocesses.

In a moving face-down mode operation, as shown in FIGS. 9A and 9B, molds106 move on a conveyor belt (not shown) with an etched cavity side 108facing downwardly. A flexible die head 110 remains stationary to scanthe bottom surface 108 of the mold 106. In this mode of operation, awave soldering tool can be modified for running the process. Forinstance, the solder supply nozzle can be modified to supply solderthrough a flexible stainless steel bellow 112 which receives a moltensolder supply from a solder pump 114. At the upper end of the bellow112, a present invention flexible die head 110 is mounted as aninterface between the bellow 112 and the mold plate 106. The desirablebenefits of the present invention novel flexible die head is fullyutilized to allow an intimate contact between the die head and with thebottom surface 108 of the mold plate 106 to be achieved. This allows allthe cavities positioned in the bottom surface 108 to be completelyfilled with the molten solder fed from the solder pump 114 under a smallpositive pressure. As shown in FIG. 9B, the bellow is slightlycompressed as the mold plate 106 scans over the flexible die head 110.The compressibility of the bellows therefore further improves thejoining between the flexible die head and the mold plate under a smallpressure. This mode of operation therefore incorporates the samedesirable manufacturing benefits of the moving face-up mold operationmethod, while an existing wave soldering tool can be modified to carryout the process.

The advantages of the present invention molten solder screeningtechnique become more apparent when the size of mold plates that can befilled is considered. The conventional injection molded solder techniquecan only fill cavities that are shorter than the length of the vacuumlink between a vacuum slot and a solder slot. The present inventionnovel method overcomes that limitation based on the fact that a vacuumlink is not required for the process. Furthermore, the present inventionflexible die head can be made wider than the width of the entire moldthat is being scanned. For example, a molten solder screening tool canhave a flexible die head that is 12" wide which is capable of scanningsubstrates (or molds) of either 12" or 8". A 12" wide flexible die headcan therefore be used to scan any molds that are smaller than 12", aslong as the carrier holding the parts has an adjustable width. Thus,even chip-sized molds of 1" or 2" widths could be scanned with a 12"tool provided that the solder slot is cut to a length less than the moldwidth. This is readily adjustable by cutting only the Teflon® tape tothe appropriate length. However, a preferred embodiment is where thelength of the flexible die head matches the mold width.

The in-situ filling of substrates with sequentially patterned layers mayhave significant applications in low-cost packaging processes. Suchapplications may include consumer electronics and automotiveelectronics. Even though the present invention technique utilizes moltensolders, which do not have resistivities as lows as copper or gold, theyshould suffice for many industrial applications that do not requirehigh-quality conductive layers. When the present invention technique isused on organic-natured circuit boards, solder materials in the range ofeutectic 63% Sn/37% Pb melting points should be used. When the presentinvention method is used on etched silicon or glass without the presenceof polymers, substantially higher temperature molten metals can be usedin forming the conductive components.

Furthermore, the present invention molten solder screening technique canbe applied to either transfer molds that are used for transferringsolder bumps to a final solder receiving surface, or to circuit boardsdirectly. Since the present invention method does not require a sealbetween a liquid solder source and a receiving layer, the flatness ofthe receiving layer is not a requirement. The flexible die head of thepresent invention and the use of its trailing edge as a wiper readilyaccommodates organic circuit boards that may have substantial warpagewithout affecting the effectiveness of wiping. Unlike the conventionalmethod of wave soldering which relies on surface tension for forming amaximum solder height, the molten solder screening technique can buildsolder structures to any desirable height by filling sequentiallydeposited and patterned layers into cavities. In most applications,these layers may remain in place for forming a multi-layered electricalredistribution structure. Some other applications may remove layersforming the depressions which receive the molten solder or metal, andthus leaving free-standing metal structures. The present inventionapparatus, in its broadest form of application, functions similarly as a3-D stereolithography apparatus capable of producing metal parts.

The present invention novel method and apparatus for forming solderbumps by a molten solder screening technique have therefore been amplydemonstrated in the above descriptions and the appended drawings ofFIGS. 1A˜9B. It should be noted that the present invention novel methodand apparatus can be advantageously used in forming solder bumps on anyelectronic structure, even though a silicon wafer has been used todemonstrate the process. The present invention novel method may furtherutilize any solder materials including those desirable lead-free soldermaterials that have been properly used recently in the semiconductorindustry.

While the present invention has been described in an illustrativemanner, it should be understood that the terminology used is intended tobe in a nature of words of description rather than a limitation.

Furthermore, while the present invention has been described in terms ofa preferred embodiment, it is to be appreciated that those skilled inthe art will readily apply these teachings to other possible variationsof the inventions.

The embodiment of the invention in which an exclusive property orprivilege is claimed are defined as follows:

What is claimed is:
 1. A method for filling a multiplicity of cavities positioned in the surface of a substrate with molten solder comprising the steps of:providing a stream of molten solder through an opening of a sheet die, passing said multiplicity of cavities in the surface of the substrate in intimate contact with said sheet die opening, said contact being adjusted such that molten solder in the stream exerts a pressure against the surface of the substrate so as to fill the multiplicity of cavities with the molten solder, and removing excess molten solder from the surface of the substrate.
 2. A method for filling a multiplicity of cavities according to claim 1 further comprising the step of providing a stream of molten solder through a die constructed of a metal sheet.
 3. A method for filling a multiplicity of cavities according to claim 1 further comprising the step of providing a stream of molten solder from a die constructed of a metal sheet having a thickness of not larger than 0.045" and a modulus of elasticity greater than 24×10⁶ psi.
 4. A method for filling a multiplicity of cavities according to claim 1 further comprising the step of providing a stream of molten solder from a die constructed of a metal sheet capable of flexing of at least 1.5 μm per inch of die length.
 5. A method for filling a multiplicity of cavities according to claim 1, wherein said removing step is carried out by wiping molten solder off the surface by a trailing edge of a die constructed of a metal sheet.
 6. A method for filling a multiplicity of cavities according to claim 1, wherein said stream of molten solder has a composition between about 58% tin/42% lead and about 68% tin/32% lead.
 7. A method for filling a multiplicity of cavities according to claim 1, wherein said multiplicity of cavities each having a depth-to-width aspect ratio between about 1:1 and about 1:10.
 8. A method for filling a multiplicity of cavities according to claim 1, wherein said stream of molten solder is provided under a pressure of not less than 2 psi.
 9. A method for filling a multiplicity of cavities according to claim 1 further comprising the step of providing a substrate made of a material having a coefficient of thermal expansion substantially similar to that of a final solder receiving material.
 10. A method for filling a multiplicity of cavities according to claim 1 further comprising the step of adjusting said contact between the multiplicity of cavities and the surface of the molten solder stream by a pressure means mounted on a flexible die head.
 11. A method for forming a multiplicity of solder bumps on the surface of an electronic device comprising the steps of:providing a substrate having a multiplicity of cavities in a top surface, providing a stream of molten solder through an elongated opening of a sheet die, passing said multiplicity of cavities in the surface of the substrate over and in intimate contact with said elongated opening of said sheet die, said contact being adjusted such that molten solder in said stream exerts a pressure against the surface of the substrate so as to fill the multiplicity of cavities with the molten solder, removing excess molten solder from the surface of the substrate and allowing the molten solder in said multiplicity of cavities to solidify, contacting and transferring said solder in said multiplicity of cavities with and to a surface of said electronic device forming solder bumps.
 12. A method for forming a multiplicity of solder bumps according to claim 11 further comprising the step of providing a stream of molten solder through a die constructed of a metal sheet.
 13. A method for forming a multiplicity of solder bumps according to claim 11 further comprising the step of providing a stream of molten solder a die constructed of a metal sheet capable of flexing at least 1.5 μm per inch of die length.
 14. A method for forming a multiplicity of solder bumps according to claim 11, wherein said removing step is carried out by wiping molten solder off the surface by a trailing edge of a die constructed of a metal sheet.
 15. A method for forming a multiplicity of solder bumps according to claim 11, wherein said stream of molten solder provided has a composition between about 58% tin/42% lead and about 68% tin/32% lead.
 16. A method for forming a multiplicity of solder bumps according to claim 11 further comprising the step of providing said stream of molten solder at a pressure of not less than 2 psi.
 17. A method for forming a multiplicity of solder bumps according to claim 11 further comprising the step of providing an electronic device of a semiconductor IC chip.
 18. A method for forming a multiplicity of solder bumps according to claim 11 further comprising the step of providing an electronic device of a semiconductor chip formed on a silicon substrate.
 19. A method for forming a multiplicity of solder bumps according to claim 11 further comprising the step of providing a substrate made of a material having a coefficient of thermal expansion substantially similar to that of silicon.
 20. A method for forming a multiplicity of solder bumps according to claim 11 further comprising the step of adjusting said contact between the multiplicity of cavities and the surface of the molten solder stream by a pressure means mounted on a die head.
 21. A method for forming a multiplicity of solder bumps according to claim 11 further comprising the step of heating said substrate and said electronic device together at a temperature not less than the solder reflow temperature for transferring said solder to the surface of said electronic device. 